We consider the propagation of the interface phonons in the active core of a quantum cascade laser. Based on the translation invariance in the interior of the active core, an implicit dispersion relation is derived. Numerical calculations show that the interface phonons in the active core can be classified as bulk phonons and surface phonons. The dispersion curves of bulk phonons form a series of subbands, and their electrostatics potential is distributed in the whole region of the active core, showing the character of Bloch wave. While the dispersion curves of the surface phonons are located in the gap between the subbands of the bulk phonons, and the electrostatic potential is localized near the interface between the active core and the waveguide layer. Our results are helpful to the design of quantum cascade lasers and other intersubband lasers.